1. Field of the Invention
The present invention relates to a device to control a 3-phase motor, and more particularly, to a device to control a 3-phase motor having an inverter and a brake.
2. Description of the Related Art
Recently, a lot of household appliances such as an air conditioner, a washing machine, and a refrigerator use a 3-phase motor controlled by an inverter to increase energy efficiency. A servo motor system designed for the household appliances generally adopts a brake integrated with a motor as an arresting gear. The brake integrated with the motor is powered by an additional power supply to be electrically insulated from a system, and is generally controlled by a relay.
FIG. 1 is a circuit diagram of a conventional device to control a 3-phase motor. As illustrated in FIG. 1, the device to control the 3-phase motor includes a 3-phase motor 100, an inverter 110, a brake power supply 120, a power supply 130, and a microcomputer 140.
A brake 101 to arrest the motor 100 and a brake coil 102 are integrated into the 3-phase motor 100. The brake 101 keeps arresting the 3-phase motor 100 as long as current is not applied to the brake coil 102. Meanwhile, the brake 101 stops arresting the 3-phase motor 100 if the current is applied to the brake coil 102.
The inverter 110 includes six transistors to switch, and a transistor operation circuit 111. The transistors are divided into three first switches Q1 through Q3 connected between respective phase terminals of the 3-phase motor 100 and a positive terminal “P” of a DC (direct current) power supply, and three second switches Q4 through Q6 connected between the respective phase terminals of the 3-phase motor 100 and a negative terminal “N” of the DC power supply. Also, each transistor is connected to a freewheeling diode Df1 in parallel.
The transistor operation circuit 111 including a gate-drive IC 112 is connected to a gate terminal of the transistor, and controls switching of the transistors. The gate-drive IC 112 is supplied with predetermined input powers Vdu, Vdv, Vdw, Vdn (or will be indicated as an inverter power) and biases the gate terminal according to an applied control signal.
The brake power supply 120 includes a switching-mode power supply (SMPS) control IC 121, a brake control relay 122, a photo coupler 123, diodes Db, Df, and a capacitor Cb. The SMPS control IC 121 in the brake power supply 120 supplies predetermined brake power to the brake coil 102 integrated with the servo motor. The brake control relay 122 switches the power supplied to the servo motor from the SMPS control IC 121 according to an external control signal. The diode Df is a freewheeling diode, and the other diode Db is used to prevent the current from flowing in a reverse direction. The photo coupler 123 is a combination of a light-emitting diode LED1 and a transistor Q7 electrically insulated from each other. As the current is applied to the LED1, the photo coupler 123 emits light and gets switched on by biasing of a base terminal of the transistor Q7.
In the brake power supply 120, if the SMPS control IC 121 outputs voltage, the capacitor Cb gets charged and the photo coupler 123 is applied with the current, which switches the brake control relay 122 on. Accordingly, the brake coil 102 is supplied with the power.
The power supply 130 includes a SMPS control IC 131, a transformer 132, a diode Dr, and the capacitor C1. The SMPS control IC 131 applies predetermined voltage to a first induction coil 133 of the transformer 132. The transformer 132 induces less voltage than the voltage applied to the first induction coil 133 to opposite terminals of a second induction coil 134 having a relatively smaller winding ratio.
The voltage applied to the second induction coil 134 is charged in a capacitor C1 via the diode Dr to prevent a counter-current. Here, the voltage charged in the capacitor C1 is supplied to the gate-drive IC 112 as an input power.
The microcomputer 140 controls operation of the 3-phase motor 100 by outputting the control signal of the gate-drive IC 112 of the inverter 110, and controls operation of the brake 101 by outputting a predetermined brake control signal to the photo coupler 123 in the brake power supply 120 and switching the LED1.
FIGS. 2A through 2C are graphs illustrating operation of a conventional circuit to control the 3-phase motor based on time. As shown in FIG. 1 and FIGS. 2A through 2C, the 3-phase motor 100 is generally arrested by the brake 101 when the power is off, or when the brake control signal is 0, e.g., 0V. In other words, if the brake control signal is 0, the brake control relay 122 is in an open state. Meanwhile, if the brake control signal is 1, the brake control relay 122 is in a closed state, loosing the brake 101. Accordingly, as illustrated in FIG. 2A and FIG. 2B, if the brake control signal is 0, the voltage between opposite terminals of the freewheeling diode Df1 (will be referred as brake voltage Vb) becomes 0. Also, the brake voltage Vb increases to a predetermined voltage when the brake control signal is 1.
FIG. 2B shows that the brake voltage Vb has a time-delay to be stabilized and loosen the brake 101, even after the brake control signal becomes 1. The time-delay is divided into a time-delay tr that it takes for the brake control relay 122 to begin operation by an operation coil, and a time-delay tb due to a bounce time decided by a specification of the brake control relay 122.
A conventional motor cannot be controlled with the brake 101 on. An axis of the 3-phase motor 100 will be distorted by force of the brake 101 holding the axis of the 3-phase motor 100, and by torque of the 3-phase motor controlled by the inverter 110. If the distortion becomes serious, the brake 101, the axis of the 3-phase motor 100, and the inverter 110 may be damaged. Hence, the output signal from the inverter 110 should be applied to each phase of the 3-phase motor 100 after the brake 101 is completely disengaged, and the brake 101 should start to operate after the output signal from the inverter 110 completely disappears. A relationship between the above-described mechanism and time is illustrated in FIG. 2B and FIG. 2C.
The conventional device to control the 3-phase motor 100 is using the brake power supply 120 additionally, other than a power supply for operation of the inverter (input power of the gate-drive IC 112) as well as using the brake control relay 122. In the relay 122, as an arresting capacity of the brake increases, the current required to control it increases. Accordingly, a size of the relay 122 and the brake coil 102 have to be increased, which has a disadvantage of complicating the circuit to control the 3-phase motor and increasing production cost.
Also, the conventional device to control the 3-phase motor 100 has low energy efficiency because the brake and the inverter keep consuming power even while the 3-phase motor 100 is arrested by the brake 101. The relay 122 causes a time-delay in operation of the 3-phase motor 100 responding to the brake control signal and the inverter control signal.